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Grower Jake Rasch demonstrates his use of string thinning on second-year peach trees while Dr. Bill Shane, right, of Michigan State University discusses orchard management Tuesday, Feb. 9, 2016, as part of the International Fruit Tree Conference in Grand Rapids, Michigan. "This made me money the first year," Rasch said of the thinner.<b> (Ross Courtney/Good Fruit Grower)</b>

Grower Jake Rasch demonstrates his use of string thinning on second-year peach trees while Dr. Bill Shane, right, of Michigan State University discusses orchard management Tuesday, Feb. 9, 2016, as part of the International Fruit Tree Conference in Grand Rapids, Michigan. “This made me money the first year,” Rasch said of the thinner. (Ross Courtney/Good Fruit Grower)

Once a tool for stone fruit, the string thinner is gaining interest with apple and cherry growers for bloom control.

Powered by a battery that fits in a fanny pack, the hand-held thinners offer a cost-saving middle ground between plucking blossoms by hand and tractor-mounted, motorized string thinners.

Michigan IFTA 2016After testing in 2015 on fraction-of-an-acre blocks, Starr Ranch Growers of Wenatchee, Washington, plans to use the hand-held thinners on a commercial scale in apples and cherries this year, Jeff Cleveringa, the orchard manager, told more than 400 attendees at the International Fruit Tree Association conference in early February in Grand Rapids, Michigan.

“It’s quite effective and it’s been working very well for us, and I think we’re going to be expanding from testing to probably doing acres of it this year to help reduce our hand-thinning bill,” Cleveringa said.

The thinners, mounted on a pole up to 6-feet long, cost less than $1,000 each, compared with more than $10,000 for the high-end, tractor-mounted models of motorized thinners. They employ floppy cords that whirl around the stick and break up bloom clusters to control fruit size and crop load.

For many years, growers of peaches, apricots and other stone fruits turned to string thinners — either motorized or hand-held — because they had few chemical thinning options, Cleveringa told Good Fruit Grower. Now, apple and cherry growers are becoming fans, too, he said.

The hand-held devices allow workers to thin with more precision, dropping more blooms from one part of the tree and less from another. The tractor-mounted versions create more uniform results, working best on flat fruit walls. Starr Ranches has cherry trees with wider spacing and cylindrical canopies, lending themselves to the control of the hand-held models.

Also, the nimbleness of the hand-held tools allow for a quick reaction, letting workers thin early, as blossoms hit the pink stage, Cleveringa said. That reduces bloom stress on trees and allows side blooms to replace any king blooms that the string thinners inadvertently knocked off.

Chemical thinners need to set on a tree for a while, leaving growers to guess on bloom timing based on weather and pollen tube models. “It’s fairly predictable, but not guaranteed,” Cleveringa said.

Cleveringa warned the conference crowd to watch their timing. Thin too late and you damage the fruit, he said, showing a slide of a scarred apple that developed from a king bloom thinned at petal fall.

“This is what happens if you’re late,” he said.

Michigan growers use string thinners, too.

“This made me money the first year,” said Jake Rasch, a sixth-generation grower, holding a thinner over his head during an association conference tour of his Kent City farm north of Grand Rapids.

While Jake Rasch demonstrated the techniques on a young Glenglo peach tree, his brother, Nick, explained that the Rasch Family Orchards sends out thinning crews in teams of four, two on a platform and two on the ground. They attach the pole thinners to an electric drill, powered by a generator.

“And you get fruit size because you get your bloom off earlier,” Nick Rasch said.

He also likes how it allows for quick adjustments compared to the uniformity of a Darwin. “You can kind of select a little bit better which limbs need some blossom thinning and which ones don’t,” he said.

Back in Washington, about 10 growers in the state are using the hand-held string thinners for apples, cherries and stone fruit, said Karen Lewis, a Washington State University Extension educator in Moses Lake, Washington.

Royal Slope grower Mike Robinson, who is also general manager of Double Diamond Fruit in Quincy, Washington, was one of the early adopters about five years ago. He wanted more control in his apricots than is offered by the tractor-mounted systems, but hand-thinning costs thousands of dollars per acre in labor.

Robinson liked the string thinners so much he began using them on apples about two years ago, determining through trials that one worker holding a string thinner can knock down 50 percent of the blossoms at the same pace as 10 hand thinners. He now owns about 15 string thinners and is still fine-tuning his practice.

String thinners are not perfect, Robinson cautioned. He still sends employees to clean up blossoms by hand in places, while sometimes the thinners tangle with trellis twine.

Also, some studies have shown that string thinning can spread fire blight, though he has had no such trouble so far himself.

“It’s like every new idea,” he said. “It takes time to make it work correctly.” •

– by Ross Courtney